a) Field of the Invention
The invention is directed to a rotating drill head with a boring bar which can be swiveled by small amounts vertical to the axis of rotation by fluidic means and serves to receive a cutting chisel, and with a pressure chamber for receiving a driven member which acts on the boring bar via a cylindrical tappet located in an eccentric, axially parallel bore hole of the boring bar.
b) Background Art
In a known construction of such a drill head, the driven member is a plate having projections and resting thereon. The tappet situated in the eccentric bore hole of the boring bar rests on the side of the plate which faces away. When acted upon in the axial direction, this tappet causes the boring bar to be stretched along its thinner wall so that the boring bar swivels toward the thicker wall and, in doing so, impresses upon the cutting chisel a circular advancing motion in the .mu. range transverse to the axis of rotation (see U.S. Pat. No. 3,007,356, FIG. 2).
Since the tappet contacts the plate eccentrically, the plate and tappet form a lever gearing which is subject to repetitive friction. Further, the tappet is acted upon obliquely as a result of this construction and accordingly experiences lateral deflections which lead to jamming of the tappet in the bore hole. Frictional and clamping forces prevent such a drill head from following slight changes in pressure whose effect does not exceed the frictional forces. When larger forces occur the cutting chisel position cannot be associated with the control pressure in a reproducible manner so that different cutting chisel positions result also when the control pressure remains the same, depending on whether it is dropping or rising.
In modern manufacturing technology there is an increasing demand in particular to produce the inner surface area of cylinders in the quickest and most accurate possible manner with respect to dimensions, so that the surface contour deviates from an exactly cylindrical surface area by a maximum of 500 to 1,000 with a tolerance in the 1.mu. range .+-.50%, depending on the path of the cutting chisel parallel to the axis of rotation and on its angle of rotation relative to the work piece. Thus, such out-of-center holes whose surface accordingly exhibits a polar and/or axial deviation from an axially parallel straight line or axially concentric circle cannot be produced by such a drill head.
For this purpose, provided that the control pressure source furnishes the necessary control pressure at the correct time and at the correct level, the mechanical adjusting or operating means must be constructed in such a way that the cutting chisel can follow the control pressure with sufficient accuracy and without hysteresis and, further, the inherent frequency of the drill head which is influenced by the rigidity or stiffness of the mechanical operating means must lie well above the control pressure frequency. Even a slight difference between these frequencies leads to resonance phenomena which render the drill head unusable above this frequency range.
To prevent troublesome frictional forces and maintain the desired rigidity of the mechanical operating means, it is known to construct the drive member and boring bar in one piece (see DE 39 29 472-A1, FIG. 1).
To this end, the drill head includes a flat rotating part having a circular cross section and divided into two portions by an eccentric recess or groove, namely into a portion forming a thin plate and a portion forming a thick plate, these portions being connected with one another by the eccentric portion formed by the groove. The relatively thin plate resulting from the groove is springing or resilient relative to the comparatively thick plate and simultaneously serves as a movable pressure seal for a compression motor, whose rigid base forms the thick plate or disk. In addition, the thin plate has a continuation which faces away and has a cylindrical outer surface area in which a piston-shaped continuation engages, the latter being constructed in one piece with the boring bar. This piston-shaped continuation forms the driven member and is securely connected with the thin plate by screws. Due to the eccentricity of the remaining portion between the thick and thin plates, a change in pressure of the pressure medium supplied to the compression motor results in a driven path in the .mu. range which is determined by the path of the driven center of gravity of the thin plate relative to the thick plate.
Although such a drill head has no structural component parts in a frictional engagement with one another and its mechanical operating members are sufficiently stiff, the manufacturing cost is considerable, as the thickness of the thin plate formed by the eccentric groove must have very accurate dimensions while taking into account the fact that with linearly increasing thickness, the increase in stiffness is greater than a linear increase, i.e. to approximately the third power. If a stiffness with a tolerance of 1% is aimed for, the thickness of the thin plate must be produced with a tolerance of approximately 0.33%. If the reference or desired thickness is 5 mm, for example, the tolerance must accordingly be approximately 162/3.mu.. It will be understood that this tolerance can be achieved with a recessing tool only by taking special steps.
In addition, such a drill head has the following disadvantage:
As is well known, the necessary mechanical work for the displacement of the cutting chisel is the product of force times distance and the force is determined by the available pressure per surface area unit multiplied by the effective surface area of the hydraulic motor. Further, it is demanded of any cutting chisel holder that the spring stiffness between the chucking or clamping region and the cutting chisel be as great as possible, e.g. one ton/mm, and the cutting chisel should be capable of displacement by the greatest possible distance, e.g. 500.mu.. Therefore, the largest possible effective surface of the hydraulic motor is desired for furnishing the necessary force. On the other hand, the working space for such drill heads for reworking or subsequent machining of inner surface areas to precise dimensions is very limited as a rule and the geometrical lo dimensions of the drill heads are subject to strict limitations.
In the drill head known from DE 39 29 472-A1, FIGS. 4 and 5, the bending springs necessary for the pivoting of the driven member already require e.g. approximately half of the available region of the cross-sectional surface of the drill head, so that there is little available effective surface for the pressure chamber, which causes a reduction in spring stiffness at a given maximum work pressure of the pressure medium.